The present application relates to an apparatus and method for analog-to-digital conversion, and more specifically to a parallel analog-to-digital converter system utilizing photonic sampling.
Analog-to-digital conversion is well known as a process in which a continuous time analog signal, which theoretically has an infinite number of values or states, is converted to a digital signal, which has a finite number of values or states. Typically, in analog-to-digital conversion, the analog signal is first sampled. The sampled analog signal is represented as a series of pulses. Each pulse has a magnitude equal to the magnitude of the analog signal at a discrete moment in time. After sampling, the discrete time signal is then quantized by rounding the value of each pulse to the closest one of a finite number of values. The resulting signal is a digital version of the analog signal.
One byproduct of analog-to-digital conversion is quantization noise. Quantization noise occurs because the magnitude of the analog signals entering a quantizer can theoretically be equal to an infinite number of values, whereas the magnitude of the rounded signals leaving the quantizer can only be equal to a finite number of values. Therefore, the quantizer causes a rounding off error, or quantization noise.
One way to reduce quantization noise is through oversampling. It is well known that to recover a sampled analog signal, the signal must be sampled at a rate greater than or equal to twice the signal frequency. Oversampling refers to sampling the signal at a rate much greater than twice the signal frequency. Increasing the sampling frequency spreads the quantization noise over a larger bandwidth because the total amount of quantization noise remains the same over the different sampling bandwidths. Thus, increasing the sampling rate relative to twice the signal frequency, or oversampling, reduces the quantization noise in the bandwidth of interest.
One ADC architecture well known in the art that uses over-sampling is the delta-sigma modulator-based ADC. A delta-sigma (xcex94xcexa3) modulator consists of an analog filter and a quantizer enclosed in a feedback loop. The filter, in conjunction with the feedback loop, acts to attenuate the quantization noise at low frequencies while amplifying the high-frequency noise. Since the signal is oversampled at a rate much greater than twice the signal frequency, a digital lowpass filter can be used to remove the high frequency quantization noise without affecting the signal.
A problem with xcex94xcexa3 modulator-based ADCs is the oversampling requirement, that is, the circuitry of the ADC must be designed to operate at a significantly higher frequency than the maximum frequency of the analog signal that is converted by the ADC. The greater the required accuracy of the xcex94xcexa3 modulator-based ADC, the larger the sampling frequency must be. Limitations in circuitry capabilities have, therefore, limited the use of the single channel xcex94xcexa3 modulator-based ADCs to relatively low signal frequencies. However, the sampling frequency may be reduced by using multiple xcex94xcexa3 modulators in parallel. An ADC using multiple xcex94xcexa3 modulators is disclosed in U.S. Pat. No. 5,196,852, xe2x80x9cAnalog-To-Digital Converter Using Parallel xcex94xcexa3 Modulators,xe2x80x9d issued Mar. 23, 1993 to I. A. Galton.
As shown in FIG. 1, Galton discloses an all-electronic ADC comprised of multiple parallel channels, each of which operates on the same analog input signal, and the outputs of which are summed to produce the overall digital output. Each channel is comprised of a multiplier 101 that multiplies its input by an internally generated sequence u(n) followed by a xcex94xcexa3 modulator 111. The output of each xcex94xcexa3 modulator 111 is filtered by a digital low pass filter 112 followed by an N-sample decimator 113. Another multiplier 121 multiplies the decimated output by another internally generated sequence v(n). The first multiplier 101 and the xcex94xcexa3 modulator 111 can be considered as mostly analog functions, while the lowpass filter 112, decimator 113, and second multiplier 121 are digital functions. The internally generated sequences are derived from a Hadamard matrix, in which the multipliers use factors of +1 or xe2x88x921. The ADC disclosed by Galton provides significant levels of ADC accuracy using a sampling frequency as low as twice the signal frequency. The ADC accuracy is increased by using additional channels.
A difficulty associated with sampling, however, is that temporal jitter in the occurrence of the sampling clock limits the performance of the ADC by causing non-uniform sampling and therefore increases the total error power in the quantizer output. If the clock jitter is assumed to contribute white noise, the total power of the error is reduced in an ADC by the oversampling ratio. Nevertheless, the clock jitter still can be a limiting factor for conversion of wideband signals.
Fortunately, sampling jitter limitations can be overcome by using photonic sampling. Photonic sampling makes use of ultra-short laser pulses with high temporal stability to sample an analog electrical input. Compared to electronic samplers, the photonic approach is capable of shorter sampling windows (sub-picosecond) and higher sampling rates, approaching 100 gigasamples per second (GSPS), and thus can sample wideband analog inputs.
A conventional photonically sampled A/D converter 200 is shown in FIG. 2. A series of optical impulses 201 from a mode-locked laser 203 are applied to an electro-optic modulator 205. The analog electrical input X(t) is also applied to the modulator 205. The optical impulses 201 sample the voltage on the modulator 205 electrodes. The resultant optical pulses 207, with intensities determined by the modulator 205 voltage, are fed to a photodetector 209. The photodetector 209 electrical output 211 is an electrical current, which can be supplied to the input of an electronic quantizer 212.
The above approach achieves very high sampling rates because the pulse repetition rate of the mode-locked laser can be 40 GHz or higher. Even higher repetition rates for the optical sampling pulses can be achieved by combining several time-delayed copies of each laser pulse.
Another example of a photonic sampled A/D converter, described by P. E. Pace and J. P. Powers of the U.S. Naval Postgraduate School in xe2x80x9cPhotonic Sampling of RF and Microwave Signals,xe2x80x9d Mar. 16, 1998, is illustrated in FIG. 3. The A/D converter of FIG. 3 uses a delta-sigma modulator architecture, which is well known for reducing quantization noise, as described by Galton, with a photonic sampler. The delta-sigma converter 300 contains a mode-locked fiber laser 302 to act as the source of sampling pulses, and two photonic samplers 304, which are Mach-Zehnder interferometric modulators. The fiber-lattice structure 306 acts as an optical integrator. The photonic samplers 304 also serve as the analog summing point at the input of the delta-sigma loop.
One difficulty associated with the photonic A/D converters discussed above is high non-linearity and noise spurs. The dynamic range of the photonic sampler limits prior A/D converters using photonic sampling techniques. For example, an analog waveform with a 5 GHz bandwidth can only be sampled to a resolution of 7.5 bits because the spur-free dynamic range of such modulators is approximately 110 dB-HZ2/3. Therefore, what is needed is an A/D converter system that can utilize optical sampling without being adversely affected by the noise and distortion from the photonic sampler.
An object of the present invention is to provide a method and apparatus for using optical sampling to provide for analog-to-digital conversion of an analog signal. An additional object of the present invention is to provide such conversion while reducing the noise and distortion associated with optically sampling an analog signal.
The photonic ADC system of the present invention overcomes the difficulties associated with photonic sampling by incorporating multiple photonic samplers in a modified parallel A/D converter architecture. In addition, the dynamic range of the photonic sampling process is improved by averaging and by canceling the common-mode laser noise with the dual-complementary photodetection. Prescribed weights can be applied digitally to the parallel channels for cancellation of sampler spurs and to compensate for non-uniformity between the samplers. Thus, the photonic A/D converter system of the present application achieves an improved resolution, as determined by the signal-to-noise and spur free dynamic range, and bandwidth over what can be achieved with a conventional combination of a photonic sampler and a separate electronic ADC.
One embodiment of the present invention provides an analog to digital converter having an analog input and a digital output, said analog to digital converter comprising: an optical pulse source; a plurality of channels, each channel comprising: an optical encoding sampler, coupled to said optical pulse source and said analog input, sampling said analog input to produce a sampled optical signal and encoding said sampled optical signal with an encoding code sequence to produce an encoded optical signal; an optical to electric converter, coupled to said optical encoding sampler, converting said encoded optical signal to an electronic signal; a quantizer, coupled to said optical to electric converter, producing a digital signal from said electronic signal; and a digital decoder, coupled to said quantizer, decoding said digital signal with a decoding code sequence to produce a decoded digital signal; and a summer, coupled to said digital decoder in each channel of said plurality of channels, digitally summing each decoded digital signal from each channel to produce said digital output. Preferably, the optical encoding sampler comprises a Mach-Zehnder interferometer to which the analog signal is applied and a directional coupler to which the encoding sequence is applied. The encoding sequence is preferable a Hadamard sequence. A Nyquist filter may be used after the optical-to-electric converter and a digital noise reduction filter may be used before the digital decoder. Preferably, a delta-sigma modulator is used for the quantizer, due to the additional noise shaping capability provided by the delta-sigma modulator architecture.
Another embodiment of the present invention is provided by an analog to digital converter having an analog input and a digital output, the analog to digital converter comprising: means for producing optical pulses; a plurality of channels, each channel comprising: means for sampling and encoding the analog input with the optical pulses, thereby producing an encoded optical signal; means for converting the encoded optical signal to an electric signal; means for quantizing the electric signal, thereby producing a digital signal; and means for decoding the digital signal with a decoding code sequence, thereby producing a decoded digital signal; and means for summing together each decoded digital signal from each channel in the plurality of channels, thereby producing the digital output. The means for sampling and encoding the analog input may comprise an optical encoding sampler circuit, an integrated optical encoding sampler, or other means that modulate a stream of optical pulses with both an analog signal and a encoding signal. The encoding signal is preferably a Hadamard sequence. The means for quantizing the electric signal may comprise a delta-sigma modulator, a flash analog-to-digital converter, successive approximation converters, or other such quantization means known in the art.
Another embodiment of the present invention is provided by a method for converting an analog input signal to a digital output signal, the method comprising the steps of: providing optical pulses; coupling the optical pulses to a plurality of converter channels; converting the optical pulses coupled to each channel converter channel to encoded optical samples of the analog input signal; converting the encoded optical samples to an electric signal in each converter channel; quantizing the electric signal to produce a quantized digital signal in each converter channel; decoding the quantized digital signal with a decoding code sequence to produce a decoded digital signal in each converter channel; and summing together all the decoded digital signals from each converter channel to produce the digital output signal.